Glioblastoma is the most common human brain malignancy, and as yet there is no effective treatment. Several studies have implicated neural progenitor and stem cells as targets for glioblastoma development. Here we demonstrate a novel mechanism by which differentiated astrocytes can be converted into progenitor state and act as cells of origin in GBM. Using adult inducible HGA GEMMs where Rb tumor suppression (TS) is abrogated along with Kras activation we demonstrate that Rb TS inactivation in adult astrocytes in vitro and in vivo, converts them into progenitor cells generating a pretumorigenic state that is susceptible to GBM progression. Upon Rb TS inactivation, cortical astrocytes dedifferentiated into cells expressing proliferation and progenitor markers while suppressing the expression of differentiation markers. These dedifferentiated cells were capable of forming spheres, self renewed and could be experimentally driven into multilineage differentiation. Conversion of astrocytes to progenitor-like cells was required to create susceptibility for the tumor to progress to a higher grade disease by Kras activation. By itself Kras was not sufficient either for tumorigenesis or for induction of progenitor properties. Thus the target cell population may depend on the specificity of astrocytes to stochastic events that arise during homeostasis. Some astrocytes proliferate in response to injury, alteration in Rb network can generate a progenitor population, susceptible for a second hit in the Kras pathway and progression into HGA. This process also provides a mechanism for origination of cancer-initiating cells which can propagate the cancer even after a cancer treatment. With the Rb network known to be altered in majority solid cancers our results may indicate novel mechanism for tumor initiation in a broad spectrum of cancers. Adhikari, A. Sullivan, T, and T. Van Dyke. Abrogation of Rb tumor suppression initiates GBM from differentiated astrocytes by driving a progenitor cell program. In preparation